Photodiodes convert light into electricity and thus can be used to detect light levels. An avalanche photodiode (APD) is a semiconductor device that can detect extremely low levels of electromagnetic radiation. Unlike a PIN photodiode, which generally produces a single electron for each photon received, an APD is constructed so that an electron dislodged by a photon will hit other atoms in the APD semiconductor lattice with sufficient velocity and energy so that additional hole-electron pairs are created by the collisions. Typically a free electron will create a number of hole-electron pairs, and the electrons from these pairs will, in turn, create additional electrons, thus creating an “avalanche” process. This multiplication of the electrons gives the APD an effective gain and allows the detection of very low light levels.
Advances in the fabrication and performance of the avalanche photodiodes have led to their use in the detection of individual photons and other short-duration events. When used in the single photon detection applications, APD's are frequently used in “Geiger” mode in which the APD is reverse biased to a voltage that exceeds its breakdown voltage. In geiger mode, some means is necessary to stop or “quench” the current flowing through the diode after each avalanche.
One method to quench the current is to limit the maximum current flowing through the diode, by means of a passive series resistor, to a low enough level that the current will spontaneously cease due to the statistical nature of the avalanche process. While using this circuitry, the minimum interval between detectable events is limited by the so-called “dead time”: the time required to turn off the diode completely and to recharge it, and any other parasitic or intrinsic capacitance associated with the diode, through the typically large current limiting resistor which results in a large RC time constant.
A so-called passive quench circuit is reverse biased through a biasing means such as a series resistance by applying a high voltage, VRB, comprised of the breakdown voltage, VBR, plus the overvoltage ΔV across the avalanche device. When an event such as a thermodynamically generated electron or impingement of a photon occurs in the case of an avalanche photodiode, the avalanche current begins to flow, the junction between the resistance means for biasing and the avalanche photodiode rises toward ΔV, and the voltage across the photodiode approaches the breakdown voltage VBR. Eventually the voltage at the junction stanches the avalanche current. The system will only reach full sensitivity when the discharge is completed and reset in the time dictated by the RC time constant which is typically long.
To shorten the resetting time, active quench circuits were developed which, for example, may use a comparator to sense the onset of an avalanche current and through the action of a monostable circuit, and apply a voltage of ΔV plus an excess voltage VX to the junction of the biasing resistor and avalanche photodiode to drive it safely below VBR and stop the avalanche current. And, after a short delay, typically applied through another monostable circuit, a switch is closed to ground from that junction to quickly recharge the intrinsic capacitance of the avalanche photodiode. Although this reduces the “dead time” by circumventing the RC time constant delay suffered by the passive quench circuits, it may introduce parasitic or intrinsic capacitance. This additional capacitance increases the charge flow through the avalanche diode and adds to the heating effect too. Additionally, the use of a traditional active quench circuit can also be disadvantageous since it creates a time delay in the circuit. Thus, the using an active quench circuit on both the anode and cathode sides of an APD is not typically desirable. Two examples of an active quench circuit are shown in U.S. Pat. Nos. 5,532,474 and 5,933,042, both incorporated herein by reference.